A method of judging the state of the tire provided on the vehicle, specifically, a method of measuring an air pressure in the tire includes a method of measuring by providing a measurement instrument on an air intake. There also is a device, which judges the air pressure of the tire without directly detecting the air pressure of the tire. For example, Patent Literature 1 discloses a measurement device provided on a road surface to measure the air pressure of the tire, which passes over a tire receiving plate. However, it is required to pass over a dedicated device for measurement of the air pressure in the device disclosed in Patent Literature 1.
On the other hand, Patent Literature 2 discloses a tire air pressure estimating device composed of a wheel speed sensor, road surface frictional state estimating means, which estimates a frictional state estimated value indicating a frictional state between the tire and the road surface from a wheel speed detected by the wheel speed sensor, tire air pressure estimating means, which estimates decrease in the air pressure of the tire based on the frictional state estimated value estimated by the road surface frictional state estimating means, and displaying means, which displays decrease in the estimated tire air pressure.